In this correspondence, South Korean researchers discuss the results of a study of the pathology of breast cancer tumor specimens, where the cancer cells are in contact with fat cells in the breast vs. glandular cells in the breast:

My understanding of the results is that cells in contact with fat cells multiplied slower than cells in contact with glandular cells. To me, this has implications for diagnosis (and treatment recommendations) as the faster dividing cells are more responsive to chemotherapy than the slower dividing cells. Who knows what cells the pathologist looked at when assigning a grade to the specimen, and whether that grade was based on an isolated group of cells that were multiplying particularly fast (or slow), or whether the grade is representative of the majority of the tumor cells?

Hopefully, this study will lead to additional studies that will help to refine the grading process for breast tumors.

That is certainly the case with liver biopsies, such as my husband had to have because he had Hep C. The biopsy only takes a snapshot of the organ because it is a small piece. I am sure it is the same with breast specimens. They do not make slides of the entire specimen, say 1 cm. apart, due to expense. I think we should probably all just assume that our tumors are heterogeneous, given that.